This invention relates to hoist vehicles having apparatus for increasing the payload while limiting the load bearing on each axle, and more particularly to hoist vehicles having a tipping or tilting frame on to which a payload container or the like may be pulled, carried and pushed off and having means for moving the frame longitudinally to optimize the position of the load relative to the axles of the truck.
Solid waste, for example, is collected in containers which when full are loaded onto a truck such as a semi-trailer and taken to a landfill where the container is emptied and returned by the truck to a waste collection point. A frame, called a tipping or tilting frame, is installed on the chassis of such a truck to facilitate the loading and unloading thereof. Conventionally, cables are utilized to pull the container onto the tilted tipping frame which is then lowered to its substantially horizontal position on the chassis, the frame being tilted by hydraulic cylinders. In U.S. application Ser. No. 921,597 filed Oct. 21, 1986, now U.S. Pat. No. 4,737,063, issued Apr. 12, 1988, a tilting frame having a chain hoist system is utilized instead of a cable so as to afford more mechanical control over the load and also facilitate unloading by pushing the container from the frame. Most states limit the over-the-road weight of trucks to 80,000 pounds for semi-trailers, however in the typical load configuration the maximum weight is reduced somewhat since the tipping or tilting frame has a pivot point near the rear of the truck bed. This placement of the frame results in the center of gravity of the load being located toward the rear of the bed to thereby place most of the weight of the load on the rear axle. However, the maximum weight bearing on the axles is limited by Federal Bridge Law to approximately 35,000 pounds on a tandem axle truck. In actual practice in order to ensure compliance with the limitation on the axle bearing weight, typical tipping frame hoist trucks have limited the weight bearing on each axle to less than 30,000 pounds taking into account the weight of the hoist system. Consequently, the maximum payload carried by such trucks is substantially less than that permitted.
Although there are suggestions in the prior art for longitudinally moving the frame of a truck, no prior art is known which attempts to optimize the location of the load relative to the axles but merely permit the load to be moved between two positions, i.e., between a transport position, and a loading/unloading position. These mechanisms typically employ telescoping hydraulic cylinders to move the frame and are generally intended for application when neither the total truck weight nor axle bearing weight is a significant consideration. Thus, the prior art mechanisms are not directed toward allowing longitudinal adjustment of the frame in order to optimize the load bearing on the axles. For example, in Corompt U.S. Pat. No. 4,755,097 it is recognized that the location of the center of gravity of the load affects the weight on the axles, but no provision is made to allow for adjustment of the center of gravity except to the one fixed position where the frame is located during transport. The pivot point during loading operations is changed in order to increase the ability to lift the load onto the truck bed while at the same time moving some of the weight off the rear axle during transport. In Lisota U.S. Pat. No. 2,621,814 a sliding bed is mounted on a tilting frame or the like for aiding in loading and unloading operations. Marlett U.S. Pat. No. 4,702,662 is somewhat similar to Lisota in this regard. In Derain U.S. Pat. No. 4,009,791 an L-shaped pivot member is mounted on the tipping frame and may be moved between forward and rearward positions on the tipping frame.
Since the load within a container or the like may not be equally distributed along the length of the container, and the lengths of containers may vary somewhat, it is important to adjust the transport position of the frame to where the weight bearing on the axles of the truck is optimized for each load transported. As aforesaid, the prior art has not proposed such a construction or attempted to offer a satisfactory solution to this problem.